This invention relates to valves for controlling fluid flow and particularly to valves which are commonly referred to as butterfly or ball valves. Butterfly valves will be used throughout the description of the invention as well as the background but it is to be understood that the invention is equally applicable to ball valves.
Butterfly valves conventionally have a resilient seal between the valve body and the butterfly which effects a seal when the butterfly is in the closed position. There are many specific designs for such seals to ensure an effective seal. For example, there are designs which employ the pressure of the fluid to force the seal into intimate contact when the valve is closed. A common denomination in such valve seals is that they are formed from some resilient material, such as rubber or plastic material. Although these materials provide the necessary resiliency to effect a good seal, these same materials create another problem.
It is, in many situations, desirable or necessary that a valve maintain a seal under extremely high temperatures such as when there is a fire either inside the piping system itself or outside the piping system in the vicinity of the valve. When fire causes the valve temperature to rise to high levels, the resilient seal of the valve can deteriorate or be destroyed so that it will no longer be effective as a seal.